To edit a picture made out of text characters (for example, a picture of the division of a register into fields, as a comment in a program), use the command M-x edit-picture to enter Picture mode.
In Picture mode, editing is based on the quarter-plane model of text, according to which the text characters lie studded on an area that stretches infinitely far to the right and downward. The concept of the end of a line does not exist in this model; the most you can say is where the last nonblank character on the line is found.
Of course, Emacs really always considers text as a sequence of characters, and lines really do have ends. But Picture mode replaces the most frequently-used commands with variants that simulate the quarter-plane model of text. They do this by inserting spaces or by converting tabs to spaces.
Most of the basic editing commands of Emacs are redefined by Picture mode to do essentially the same thing but in a quarter-plane way. In addition, Picture mode defines various keys starting with the C-c prefix to run special picture editing commands.
One of these keys, C-c C-c, is pretty important. Often a
picture is part of a larger file that is usually edited in some other
major mode. M-x edit-picture records the name of the previous
major mode so you can use the C-c C-c command
(picture-mode-exit) later to go back to that mode. C-c C-c
also deletes spaces from the ends of lines, unless given a numeric
argument.
The special commands of Picture mode all work in other modes (provided the `picture' library is loaded), but are not bound to keys except in Picture mode. The descriptions below talk of moving "one column" and so on, but all the picture mode commands handle numeric arguments as their normal equivalents do.
Turning on Picture mode runs the hook picture-mode-hook
(see section Hooks).
Most keys do the same thing in Picture mode that they usually do, but
do it in a quarter-plane style. For example, C-f is rebound to
run picture-forward-column, a command which moves point one
column to the right, inserting a space if necessary so that the actual
end of the line makes no difference. C-b is rebound to run
picture-backward-column, which always moves point left one
column, converting a tab to multiple spaces if necessary. C-n and
C-p are rebound to run picture-move-down and
picture-move-up, which can either insert spaces or convert tabs
as necessary to make sure that point stays in exactly the same column.
C-e runs picture-end-of-line, which moves to after the last
nonblank character on the line. There is no need to change C-a,
as the choice of screen model does not affect beginnings of
lines.
Insertion of text is adapted to the quarter-plane screen model through
the use of Overwrite mode (see section Minor Modes). Self-inserting characters
replace existing text, column by column, rather than pushing existing text
to the right. RET runs picture-newline, which just moves to
the beginning of the following line so that new text will replace that
line.
Picture mode provides erasure instead of deletion and killing of
text. DEL (picture-backward-clear-column) replaces the
preceding character with a space rather than removing it; this moves
point backwards. C-d (picture-clear-column) replaces the
next character or characters with spaces, but does not move point. (If
you want to clear characters to spaces and move forward over them, use
SPC.) C-k (picture-clear-line) really kills the
contents of lines, but does not delete the newlines from the
buffer.
To do actual insertion, you must use special commands. C-o
(picture-open-line) creates a blank line after the current line;
it never splits a line. C-M-o (split-line) makes sense in
Picture mode, so it is not changed. C-j
(picture-duplicate-line) inserts below the current line another
line with the same contents.
To do actual deletion in Picture mode, use C-w, C-c C-d
(which is defined as delete-char, as C-d is in other
modes), or one of the picture rectangle commands (see section Picture Mode Rectangle Commands).
Since "self-inserting" characters in Picture mode overwrite and move point, there is no essential restriction on how point should be moved. Normally point moves right, but you can specify any of the eight orthogonal or diagonal directions for motion after a "self-inserting" character. This is useful for drawing lines in the buffer.
picture-movement-left).
picture-movement-right).
picture-movement-up).
picture-movement-down).
picture-movement-nw).
picture-movement-ne).
picture-movement-sw).
picture-movement-se).
Two motion commands move based on the current Picture insertion
direction. The command C-c C-f (picture-motion) moves in the
same direction as motion after "insertion" currently does, while C-c
C-b (picture-motion-reverse) moves in the opposite direction.
Two kinds of tab-like action are provided in Picture mode. Use
M-TAB (picture-tab-search) for context-based tabbing.
With no argument, it moves to a point underneath the next
"interesting" character that follows whitespace in the previous
nonblank line. "Next" here means "appearing at a horizontal position
greater than the one point starts out at." With an argument, as in
C-u M-TAB, this command moves to the next such interesting
character in the current line. M-TAB does not change the
text; it only moves point. "Interesting" characters are defined by
the variable picture-tab-chars, which should define a set of
characters. The syntax for this variable is like the syntax used inside
of `[...]' in a regular expression--but without the `['
and the `]'. Its default value is "!-~".
TAB itself runs picture-tab, which operates based on the
current tab stop settings; it is the Picture mode equivalent of
tab-to-tab-stop. Normally it just moves point, but with a numeric
argument it clears the text that it moves over.
The context-based and tab-stop-based forms of tabbing are brought
together by the command C-c TAB (picture-set-tab-stops).
This command sets the tab stops to the positions which M-TAB
would consider significant in the current line. The use of this command,
together with TAB, can get the effect of context-based tabbing. But
M-TAB is more convenient in the cases where it is sufficient.
It may be convenient to prevent use of actual tab characters in
pictures. For example, this prevents C-x TAB from messing
up the picture. You can do this by setting the variable
indent-tabs-mode to nil. See section Tabs vs. Spaces.
Picture mode defines commands for working on rectangular pieces of the text in ways that fit with the quarter-plane model. The standard rectangle commands may also be useful (see section Rectangles).
picture-clear-rectangle). With argument, delete the text.
picture-clear-rectangle-to-register).
picture-yank-rectangle). With argument,
insert instead.
picture-yank-rectangle-from-register).
The picture rectangle commands C-c C-k
(picture-clear-rectangle) and C-c C-w
(picture-clear-rectangle-to-register) differ from the standard
rectangle commands in that they normally clear the rectangle instead of
deleting it; this is analogous with the way C-d is changed in Picture
mode.
However, deletion of rectangles can be useful in Picture mode, so these commands delete the rectangle if given a numeric argument. C-c C-k either with or without a numeric argument saves the rectangle for C-c C-y.
The Picture mode commands for yanking rectangles differ from the
standard ones in overwriting instead of inserting. This is the same way
that Picture mode insertion of other text differs from other modes.
C-c C-y (picture-yank-rectangle) inserts (by overwriting)
the rectangle that was most recently killed, while C-c C-x
(picture-yank-rectangle-from-register) does likewise for the
rectangle found in a specified register.